Gas separation membrane

ABSTRACT

A membrane suitable for separating a gas from a gas mixture comprising a non cross-linked PVAm having a molecular weight of at least Mw 100,000 carried on a support wherein after casting onto the support, said PVAm has been heated to a temperature in the range 50 to 150° C., e.g. 80 to 120° C.

RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/147,550, which is the U.S. National Phase of InternationalApplication PCT/GB2010/000174, filed Feb. 2, 2010 designating the U.S.,and published in English as WO 2010/086630 A1 on Aug. 5, 2010, whichclaims priority to U.S. Provisional Application No. 61/150,810 filedFeb. 9, 2009 and United Kingdom Patent Application No. GB 09016699.9filed Feb. 2, 2009.

FIELD OF THE INVENTION

This invention relates to a membrane for separating gases from gasmixtures, preferably carbon dioxide from gas mixtures containing thesame and to a process for the production of the membrane and use of themembrane to separate gases. In particular, the invention provides amembrane formed from a supported polyvinylamine (PVAm).

BACKGROUND OF THE INVENTION

Scientists have been investigating ways of separating components ofindustrial gas streams for many years. Recently, with the climatechanges being observed due to carbon dioxide emissions, ways ofseparating carbon dioxide from gas streams to try to reduce the impactsof global warming have been widely researched.

In general, gases such as carbon dioxide are separated from gas mixtureswith, for example, methane, nitrogen and/or carbon monoxide byreversible absorption methods employing various chemical and/or physicalsolvents, or by reversible adsorption in beds of adsorbents (e.g.activated carbon). As conventional processes for treating carbon dioxideare highly energy consuming and depend on the use of additionalchemicals, the cost as well as the increased demand for environmentalprotection enforce more efficient separation processes to evolve.Membrane technology is such a new separation technique. Membrane modulesalso significantly reduce weight and space requirements of separationequipment.

One option for membrane separation is the use of a facilitated transportmembrane, also known as a supported liquid membrane (SLM) with mobilefacilitated transport carriers. These have been studied for over twodecades and are known to have both high permeability of gases and highselectivity. However, for the SLM membranes serious degradationproblems, such as evaporation of solution and deactivation of complexingagent (carrier), have restricted their further development andapplication. Facilitated transport membranes with fixed carrier, such asthe PVAm blend membrane claimed herein are therefore favoured.

Other alternatives to facilitated blend membranes are however known. InJ. Membrane Science 163 (1999) 221-227, the separation and recovery ofcarbon dioxide is achieved using polyethyleneimine/polyvinylalcoholmembranes. Such membranes are however very dense and they thereforepossess poor permeance. In this regard, permeance is a measure of theflow of a gas through the membrane. High permeance implies the abilityto separate large volumes of gas with a reduced membrane area.

In U.S. Pat. No. 4,690,766, a modified polysulphone semi permeablemembrane is disclosed for use in reverse osmosis and ultrafiltration.

In EP-A-1900419 a mixture of PVAm and polyvinyl alcohol (PVA) is used asa gas separating membrane. The PVAm exemplified is of very low molecularweight and is thus used on a support with relatively low molecular weighcut off (MWCO).

In WO2005/089907, a support coated with a cross-linked PVAm is used as acarbon dioxide separating membrane. The cross-linking agent ammoniumfluoride is used to ensure the cross-linking occurs. This membranesuffers however, from a decrease in flux, in particular at higherpressures, when high molecular weight cut off (MWCO) porous supports,e.g. those of MWCO 50,000 or higher, are used. This is believed to becaused by a compaction of the selective membrane layer which may resultin a “filling-in” of the pores of the support structure.

There remains a need therefore to design further gas separatingmembranes which do not suffer from the problems highlighted above butwhich also possess excellent target gas selectivity and high permeance.The inventors have surprisingly found that certain membranes, e.g. a gasseparation membranes comprising polyvinylamine exhibit excellentseparation properties, excellent mechanical properties and are verystable. Moreover, certain membranes do not suffer from pore blockages(filling in) which may occur with other supported membranes.

The inventors have made the following remarkable observations inrelation to gas separation membranes:

1. That as an alternative to cross-linking using an externalcross-linking agent, thermal treatment of the formed membrane can bebeneficial to the permeance and selectivity of the membrane

2. That the use of a higher molecular weight PVAm polymer can improvepermeance and selectivity of a membrane relative to lower molecularweight materials;

3. Using higher molecular weight PVAm polymers enables the use of higherMWCO supports which in turn can enhance the permeance and selectivity ofthe membrane;

4. That cross-linking the PVAm using an external cross-linking agent mayreduce the permeance and selectivity of the membrane, especially forhigh molecular weight membranes which are usable without cross-linking;

5. That for some high molecular weight PVAm containing membranes neithercross-linking nor thermal treatment is needed at all;

6. That careful selection of the solvent used to cast the membrane canenhance permeance and selectivity of the membrane;

7. That control of the pH during casting of the membrane is critical tomaximising permeance and selectivity of the membrane;

8. That pre-treatment of the PVAm polymer used to manufacture themembrane can improve the permeance and selectivity of the membrane;

9. That the use of a high carbon dioxide permeance polymer layer (suchas polydimethylsiloxane, polyvinyl alcohol or chitosan layer) betweenthe membrane and support gives the membrane much higher mechanicalstrength without damaging the selectivity and permeance of the membrane.

The inventive membranes of the invention may comprise at least one,preferably a combination of the inventive features mentioned above. Theinvention encompasses any (viable) combination of these features.

SUMMARY OF INVENTION

Thus, viewed from one aspect the invention provides a membrane suitablefor separating a gas from a gas mixture comprising a non cross-linkedPVAm having a weight average molecular weight (Mw) of at least 50,000,preferably at least 100,000, carried on a support wherein after castingonto the support, said PVAm has been heated to a temperature in therange 50 to 150° C., e.g. 80 to 120° C.

Viewed from another aspect the invention provides a membrane suitablefor separating a gas from a gas mixture comprising a non cross-linkedPVAm having a molecular weight of at least Mw 50,000, preferably atleast 100,000, carried on a support having a MWCO of at least 20,000,preferably at least 50,000.

Viewed from another aspect the invention provides a membrane suitablefor separating a gas from a gas mixture comprising a non cross-linkedPVAm having a molecular weight of at least Mw 50,000, preferably atleast 100,000 carried on a support.

Viewed from another aspect the invention provides a membrane suitablefor separating a gas from a gas mixture comprising a PVAm having amolecular weight of at least Mw 50,000, preferably at least 100,000carried on a support wherein the membrane is non-strengthened.

Viewed from another one aspect the invention provides a membranesuitable for separating a gas from a gas mixture comprising a PVAmcarried on a support comprising polytetrafluoroethylene (PTFE),polypropylene (PP), sulphonated polysulfone (PSf), polyvinylidenefluoride (PVDF), and related block copolymers, polyimide (PI), polyetherimide (PEI), aliphatic polyamides, polyetheretherketone (PEEK), orpolyphenylene oxide (PPO).

Viewed from another aspect the invention provides a membrane suitablefor separating a gas from a gas mixture comprising a PVAm carried on asupport wherein a carbon dioxide permeable layer separates the PVAm fromthe support, e.g. a layer of polydimethylsiloxane (PDMS).

Viewed from another aspect the invention provides a process for theformation of a membrane suitable for separating a gas from a gas mixturecomprising:

(I) obtaining a PVAm;

(II) hydrolysing said PVAm under conditions of acid or base to form apretreated PVAm;

(III) forming a solution of said pretreated PVAm;

(IV) casting said solution, e.g. on a porous support, to form acomposite membrane; and optionally

(V) thermal treating or cross-linking said membrane.

Viewed from another aspect the invention provides a process for thepreparation of a membrane suitable for separating a gas from a gasmixture comprising:

(i) forming a solution comprising polyvinylamine in a solvent

(ii) casting said solution, e.g. on a porous support, to form acomposite membrane; and optionally

(iii) thermal treating said membrane

wherein said solvent comprises methanol, ethylene glycol, formamide,mixtures thereof or mixtures of one or more of said solvents with water.

Viewed from another aspect the invention provides a process for thepreparation of a membrane suitable for separating a gas from a gasmixture comprising:

(A) forming a solution comprising polyvinylamine in a solvent;

(B) casting said solution, e.g. on a porous support, to form a compositemembrane; and optionally

(C) thermal treating or cross-linking said membrane.

wherein the pH of the casting solution is at least 6.

Viewed from another aspect, the invention provides use of a membrane ashereinbefore defined or produced by a process as hereinbefore defined inthe separation of a gas from a gas mixture, e.g. in separating carbondioxide from a mixture containing the same, e.g. in biogas upgrading.

DEFINITIONS

The combination of the membrane of the invention carried on the supportis called a composite membrane herein.

Unless otherwise stated, a high/higher molecular weight PVAm polymer isone in which the MW is at least 100,000.

Unless otherwise stated, a high/higher molecular weight cut-off supportis one in which the MWCO is at least 50,000.

The word “non cross-linked membrane” is used herein to denote that noexternal cross-linking agent has been used to cross-link the membrane.

Thermal treatment of the composite membrane means exposing the compositemembrane to heat of at least 50° C.

By microporous support is meant a support having pores sizes of 0.10 to10 μm.

By non-strengthened is meant that the PVAm membrane is neithercross-linked nor thermally treated.

DETAILED DESCRIPTION OF INVENTION

Support

Gas separating membranes can typically take two forms, supported orunsupported. The present membranes are carried on a support. As notedbelow, the support can be in the form of a flat sheet or a hollow fibresupport. Both these support types are covered in this invention.

Suitable supports are known in the art and most are ones which areporous to the gas being transported. Suitable supports includepolyethersulfone (PES), polytetrafluoroethylene (PTFE), polypropylene,sulphonated polysulfone, polyvinylidene fluoride, polyacrylonitrile(PAN) and related block copolymers, cellulosics such as celluloseacetate (CA), polyimide, polyether imide (PEI), aliphatic polyamides,polyetheretherketone (PEEK), polyphenylene oxide (PPO) and polysulfone(PSf). Such supports are available commercially from suppliers such asOsmonics. In a preferred embodiment the support is PSf, especially wherethe support is in the form of a flat membrane. Where the support is ahollow fibre both PSf and especially PPO are preferred.

Most of these supports are typically ultrafiltration membranes where thesize of the pores in the membrane is of the order of 20 to 1000Angstroms although it is more common to express pore sizes in terms ofmolecular weight cut off values.

In some embodiments of the invention, it is also within the scope of theinvention to employ microporous support structures. Such supports havemuch bigger pores sizes, e.g. 0.10 to 10 μm making gas transport therethrough very rapid. It is not normal to express pore sizes of thesesupports in MWCO terms but in this invention, microporous supports areconsidered to have MWCO values of greater than 100,000.

Microporous supports can be formed from any suitable material includingthose mentioned above in connection with ultrafiltration membranes andinorganic materials such as ceramics (alumina, zirconium oxide), glassmembranes such as silica and so on. These can be prepared by sintering,sol gel or leaching techniques known in the art.

Conventionally, it has been assumed that the use of these microporoussupports in gas separation membranes using PVAm was not possible as thepores of the support are so large that the PVAm will simply collapseinto the pores. As noted in more detail below, the inventors have solvedthis particular problem by utilising high molecular weight PVAm polymerswhich have been found not only to possess excellent permeance andselectivity but also excellent mechanical strength. The mechanicalstrength of these membranes allows them to be used without the problemof filling in even when the pores in the support material are micronsized.

The molecular weight cut off (MWCO) of the support is preferably kept ashigh as possible. MWCO is essentially a measure of the pore size in asupport with larger MWCO values representing higher pore sizes. The MWCOin this invention is preferably more than 20,000, e.g. at least 35,000,preferably more than 50,000, more preferably at least 60,000, especiallyat least 75,000. In a highly preferred embodiment the MWCO is at least100,000. In fact, the invention enables the use of supports having MWCOof up to 300,000, e.g. 60,000 to 300,000. In one embodiment, the MWCOmay be less than the molecular weight Mw of polymer cast on top. This isnot however essential.

The problem here is that as MWCO increases, the size of the pores in thesupport also increases. This leads to the problem of filling in, wherethe membrane on the support collapses into the pores. There hastherefore been a limit on the MWCO of the support as this cannot be sohigh as to cause the filling in problem. In the prior art, the MWCO ofthe exemplified support is typically no more than 50,000.

It has been surprisingly found that when a membrane of the invention wasprepared using a PVAm of high molecular weight, the problem of “fillingin” is minimised even if using a high molecular weight support. Thisthen allows the use of a high MWCO support and can therefore lead to animprovement in permeance and selectivity.

Without wishing to be limited by theory, when using porous supports withlarger pores, whether ultrafiltration or microfiltration supports, theincreased pore size not only decreases the mass transfer resistancetowards a gas to be separated but also changes the support separationmechanism itself. An ultrafiltration membrane with low pore size (lowMWCO) may present selectivity towards nitrogen, for example, via Knudsendiffusion and not towards carbon dioxide.

As noted in more detail below, using a high Mw PVAm allows the use ofporous support with larger pores and consequently low mass transferresistance towards the gas molecules separated by the PVAm membranewithout affecting mechanical stability.

In a preferred embodiment the support can have a porous lower layer witha thin dense top layer. By dense is meant that there are no pores in thedense top layer.

The dense top layer is preferably no more than 60 nm in thickness, e.g.around 40 nm or less in thickness. It is however within the scope of theinvention for the dense layer to have a greater thickness e.g. 100 to1000 nm, such as 200 to 700 nm, e.g. 600 nm

Supports with dense top layers are preferably hollow fibre supports andideally can be formed from PPO. The dense top layer is formed during thespinning process.

Polyvinylamine (PVAm)

The weight average molecular weight (Mw) of the PVAm polymer used inthis invention can range from 10,000 to 3,000,000, e.g. 20,000 to750,000 such as 30,000 to 500,000.

The weight average molecular weight (Mw) of the PVAm is at least 50,000.Preferably, the Mw of the PVAm polymer is at least 100,000, morepreferably at least 150,000, especially at least 200,000. In someembodiments the Mw can be at least 250,000, even at least 300,000.

It has been found that using higher Mw gives the membrane strength. Thisallows the use of a support with a much higher MWCO. It is a specificfeature of the invention therefore to use a PVAm polymer having a Mw ofat least 100,000 with a support having a MWCO of at least 60,000.

It has also been surprisingly observed that even when using a highermolecular weight PVAm, this does not result in a reduction in permeanceor selectivity. The use of a higher Mw PVAm polymer means that theactual membranes used will tend to be denser than membranes formed fromlower Mw polymers. Surprisingly, the inventors have found that even athigher densities the permeance values of the membranes remain very highand the gas selectivity is good.

A further benefit of the use of a higher Mw PVAm membrane concerns wateruptake. The higher Mw PVAm membrane has more densely packed molecularchains meaning more densely packed amino groups. This leads to greaterwater uptake in comparison to lower Mw PVAm membranes which promotesreactivity of the amino groups to, inter alia, carbon dioxide.

The skilled man might also expect that increased water uptake would leadto membrane swelling and hence lower permeance values as thickermembranes are obviously harder for gases to cross. However, any swellingwhich does occur is limited and counter balanced by the increase incarbon dioxide transfer which the higher uptake of water facilitates.

The combination therefore of higher molecular weight PVAm polymermembranes with high MWCO supports provides composite membranes withexcellent properties.

A further benefit of the use of higher Mw membranes is their ability towithstand greater pressures. The membranes of the prior art areconventional used at low gas pressures. Flue gases from industrialplants can however be at relatively high pressures, e.g. up to 15 barsand ideally any membrane would be able to carry out gas separation onsuch higher pressure gases. In particular, it is preferred that thepermeance and selectivities obtained at higher gas pressures are notreduced (or not significantly reduced) relative to operation at lowerpressures. It is a further feature of this invention that the membranesclaimed are able to handle gases which are under pressure, e.g. at apressure of up to 20 bars, e.g. up to 15 bars, such as 2 to 15 bars or 2to 10 bars.

As noted above, in WO2005/089907, the inventors teach the cross-linkingof the PVAm using ammonium fluoride. The inventors have surprisinglyfound that the cross-linking step for high Mw PVAm (340000 Mw) actuallyleads to a reduction in permeance and selectivity as it causesdensification of the top layer of the membrane making it harder for thecarbon dioxide to come into contact with the amine groups in thepolymer. For a lower molecular weight PVAm polymer however,cross-linking is essential to provide a membrane with sufficientstrength that it will not “fill in” the pores in the support.

The inventors have found that when using a higher Mw PVAm membrane, therequirement to cross-link using a cross-linking agent is no longerpresent as the higher Mw provides the membrane with sufficient strengththat the filling in problem is overcome. Also, despite the use of ahigher Mw PVAm causing an overall densification of membrane relative toa lower Mw PVAm membrane, the inventors have surprisingly not observedany reduction of permeance or selectivity caused by the use of a higherMw PVAm. In fact the opposite observation is made and the membranesactually perform better than cross-linked counterparts. It is especiallypreferred therefore if the membrane of the invention is not cross-linkedusing an external cross-linking agent.

The inventors have found however that thermal treatment of the compositemembrane can provide advantageous properties, especially where themembrane will operate at elevated gas pressures, e.g. above 10 bars.

By thermal treatment is meant exposing the composite membrane (i.e.membrane on the support) to heat to induce strength therein. Suitablethermal treatment conditions encompass heating to 50 to 150° C., e.g. 80to 120° C., especially 90 to 110° C. This thermal treatment step is notregarded as being a cross-linking step as no external cross-linkingagent is employed however it does impart additional strength to themembrane, perhaps by encouraging intermolecular interaction betweenpolymer chains and between the PVAm layer and the porous support.

Thermal treatment has been found to be better than ammonium fluoride asthe permeance values and selectivities of otherwise identical compositemembranes are better than ammonium fluoride cross-linked analogues.

It will be clear that thermal treatment of the PVAm membrane takes placewhen this is supported. Without wishing to be limited by theory, it isbelieved that the thermal treatment step also modifies the support thusallowing improved permeance values. It may be that the interactionbetween the support and the dense layer of PVAm is improved.

In some embodiments of the invention, particularly when the membrane isfor operation at lower gas pressures (e.g. less than 10 bars), the PVAmmembrane can be neither cross-linked nor thermally treated. This will betermed “non strengthened” herein. It has previously been accepted thatsome form of membrane strengthening is essential to provide a membraneof sufficient strength however that is not the case where a highermolecular weight PVAm polymer is employed.

As noted above, PVAm polymers are available commercially and can besupplied in their hydrochloride salt form although the skilled chemistwill appreciate that in reality PVAm is a copolymer with an equilibriumexisting between the ammonium salt form and free amino form of thepolymer. These are named protonic form and basic form and theequilibrium is obviously pH dependent.

PVAm is normally supplied in 90%+ hydrolysed form (i.e. at least 90+% ofthe polyvinyl amide groups are hydrolysed to amino groups)—more than 90%is in PVAm and 10% is in polyvinylformamide form. These PVAm polymersstill therefore contain significant levels of polyvinylformamidepolymer.

Until now, these commercial polymers have been used as supplied by thesupplier however the inventors have found that permeance and selectivityresults can be improved if the PVAm polymer is hydrolysed prior tocasting. This additional hydrolysis step can be carried out under acidor base conditions using strong or weak acids or bases This reactionwill typically take place in aqueous solution. Acidic hydrolysis ispreferred.

The necessary reaction may involve multiple steps. It is preferred ifthe PVAm polymer is firstly re-precipitated from a suitable solvent,e.g. acetone or acetone and ethanol mixture. It can then optionally bewashed, filtered and dried normally until a constant weight is achieved.The precipitate is then re-dissolved in distilled water. This procedurecan be repeated several times.

The resulting polymer solution can be hydrolysed in acid or base butpreferably by acidic hydrolyse, e.g. in presence of HCl. The HCl used istypically quite strong, e.g. of the order of 2 to 10 M, e.g. 3 to 7Msuch as 5M solution. The PVAm can then be re-precipitated in the form ofPVAm.HCl (protonic form). The material which forms is of higher purityand performs better in the membranes of the invention than materialssupplied commercially. It is preferred therefore to minimise thepolyvinylamide content in the claimed polymers, e.g. to less than 5 wt%, especially less than 1 wt %

In some embodiments, it is preferred that the PVAm is in the form of asalt, e.g. a chloride salt as the presence of ions is believed to createpolar sites within the membrane. This enhances gas separation betweennon polar and polar gases. As noted further below however, pHmanipulation is an important part of the casting process and affects thepH of the formed membrane.

Other Membrane Components

It is preferred if the membranes of the invention consist essentially ofPVAm, i.e. PVAm forms the only material used in the membrane other thanminor levels of any necessary additives such as stabilisers,anti-oxidants or residuals solvents etc.

Support Formation

The support can be in the form of a flat sheet or hollow fibre.Techniques for making these supports are known in the art.

The hollow fibre process will normally involve dissolving the supportmaterial in a suitable solvent to form a solution and then spinning thesolution to form hollow fibres. In the spinning process, the supportsolution is fed by the force of a pump to a spinneret and subsequentlyextruded. A bore liquid is passed through the centre of the spinneret toensure that the fibres which form are hollow. The fibres pass out of thebase of the spinneret and eventually into a coagulation bath. There ishowever, an air gap between the base of the spinneret and thecoagulation bath. The presence of an air gap allows solvent evaporationand also allows the fibres to stretch and straighten under their ownweight. This hollow fibre spinning technology is well known to theskilled person.

It will be appreciated that where there is a hollow fibre the membranecan be formed on the outside or inside of the fibre (although preferablynot both). External coating can be performed simply using spraying ordip coating in a PVAm solution. Internal coating of hollow fibresinvolves circulating a PVAm solution inside the hollow fibre lumen,followed by drying as in the case of outside dip coating. The procedureis repeated until a thin defect free layer of PVAm is formed. Coatinginternally is preferred.

In a preferred embodiment the support can have a porous lower layer witha thin dense top layer. Supports with dense top layers are preferablyhollow fibre supports and ideally can be formed from PPO or PSf. Thedense top layer is formed during the spinning process.

Composite Membrane Formation

The first stage in the formation of the composite membrane of theinvention involves casting a solution of the PVAm polymer onto thesupport. The support can typically be in the form of a flat sheet orbundle of hollow fibres. Casting of the solution is carried out usingknown techniques. Various options are available for coating supportswith thin films and these include dip coating, vapour deposition, spincoating, and spray coating. These techniques will be deemed to be“casting” according to the invention.

Where the support is a hollow fibre the term casting will typically meandipping or spray coating of a hollow fibre support. Where the membraneis located within the hollow fibre the term casting covers the processdescribed above.

The solution of PVAm is typically aqueous but it has been found thatother solvents can actually offer the formation of composite membraneswith even better permeance and selectivity. This finding is new andforms a further aspect of the invention.

Other solvents which can be employed include methanol, ethylene glycoland formamide (HCONH₂) or mixtures of these solvents with each other orwith water. It is important that the solvent or solvent mixture employeddissolves the polymers in question. Preferred solvents include amethanol/water mixture, ethylene glycol/water and formamide/watermixture, e.g. containing at least 60 wt % formamide. The combination ofmethanol and water is particularly advantageous when forming flat sheetmembranes. It has been observed, for instance that the methanol watercombination is slightly less hydrophilic than water alone and thereforeinteracts more favourably with the support allowing easier and betterfilm formation.

The use of ethylene glycol and formamide slows down the solventevaporation time which has also been found to enhance the quality of theformed film. Ethylene glycol is of particular interest where the supportis a hollow fibre.

The concentration of the PVAm polymer in the solution may range from0.01 to 20 wt %, preferably 0.05 to 10 wt %. Ideal membranes have beenformed using concentrations of around 0.1 to 2 wt %, e.g. 1 wt %.

To ensure dissolution and thorough mixing, stirring and sonication canbe used at this point. The solution can also be filtered.

The inventors have realised that depending on the solvent which ischosen, the pH of the solution from which the membrane is cast maydiffer. Formamide is a basic solvent whereas methanol and water aremildly acidic. The inventors have found that pH control of this solutionis crucial to the manufacture of a membrane with high permeance and/orselectivity and this is a new finding and forms a further aspect of theinvention. The PVAm polymer possesses numerous amino groups that canexist either in uncharged amino form or as the ammonium salt dependingon the pH.

The actual pH of solution from which the membrane is cast can vary overquite wide limits depending on the solvent. As noted above, the chlorideatoms present in a hydrochloride salt might enhance separation of polarand non polar gases and hence in some embodiments of the invention thepH may be in the range 2 to 6.5, e.g. 3 to 5. It is preferred however ifthe pH of the solution is slightly more basic as this has been found toincrease permeance in the formed membrane. For example pH values of 6 to12 are preferred, e.g. 7 to 11.

It is an embodiment of the invention therefore for the casting solutionof PVAm that the pH of the solution is carefully chosen depending on thesolvent. This can be achieved by using some of the solvents mentionedabove or can be achieved in by adding acid or base or by suitablebuffering solutions. Such an acid or base will be one which does nototherwise degrade the membrane/PVAm and will be easily chosen by theskilled man. The use of buffer solutions is of particular importancehere.

The control of pH is also important in membrane formation control, inparticular in thickness control. Depending on the pH used, the viscosityof the casting solution varies. At pH 2, the PVAm is present mostly inprotonic form.

The solution viscosity of fully protonated PVAm.HCl is high due to thehigh cationic charge density on the polymer backbone. The differentsolvents such as water, ethylene glycol, formamide, methanol, and anymixture of these provide different pH values and different volatilitiesand the quality of very thin films can be affected by drying conditions,especially for hollow fiber membrane coatings. The viscosity of castingsolution and hence its pH is thus an important factor in membranepreparation on porous supports as factors such as solutionconcentration, solvent, pH and charge density of polymer affect membraneformation.

The thickness of the formed membrane will vary depending on theconcentration of the solute with higher concentration solutions givingthicker membranes. Thickness can be adjusted however using a castingknife.

The thickness of the actual membrane of the invention may be in therange 100 nm to 100 μm, preferably 250 nm to 10 μm, especially 300 nm to5 μm. Thin membranes tend to have higher permeance values but are alsoless strong.

The thickness of the support on which the membrane can be carried canvary although this may be of the order of 50 to 500 μm, e.g. around 200μm. It will be appreciated however that this invention covers both theuse of a flat membrane as well as the use of a hollow fibre support.When the support is a hollow fibre support the thickness of the supportis regarded as the wall thickness of hollow fibre.

After formation of the membrane on the support, the solvent is removed,e.g. by evaporation. This can be achieved using gentle heat ifnecessary, e.g. less than 50° C.

To avoid any possible loss of membrane forming material into the supportit is normal if there is a reasonable difference between the averagemolecular weight of the PVAm and the molecular weight cut-off of thesupport structure. Such a difference may be larger than about 10,000,such as larger than about 15,000, for example larger than about 20,000,especially more than 50,000.

The formed membrane can then be cross-linked if desired. Cross-linkingcould be effected chemically using cross-linking agents such asglutaraldehyde or ammonium fluoride. As noted above however it ispreferred if the membrane is not cross-linked.

It is also at this stage of the manufacturing process that thermaltreatment of the membrane can be effected.

The resulting membrane acts as a fixed site carrier (FSC) for gas, e.g.carbon dioxide, transport due to the high concentration of amino groups.

As noted below in the Examples, the membranes of the invention haveexcellent mechanical strength evidence by the fact that a very thin, noncross-linked membrane (˜1 μm) can be formed on a porous support withhigh MWCO (˜50,000) and can resist high pressure without a ‘filling-in’problem (at least 15 bar).

A further solution to the problem of ‘filling-in’ is to use anadditional porous support layer on the support to carry the membrane andprevent it from collapsing into the pores of the support. The inventorshave found that various gas permeable layers can be used in this regardsuch as polydimethylsiloxane, chitosan and polyvinyl alcohol can fulfilthis role. Moreover, by using an additional porous supporting layer, theinventors enable the combination of PVAm with all manner of microporoussupports which would otherwise suffer from collapse into the relativelymassive pores of a microporous membrane. T. Kouketsu, S. Duan, T. Kai,S. Kazama, K. Yamada, PAMAM dendrimer composite membrane for CO2separation: Formation of a chitosan gutter layer, J. Membr. Sci., 287,(2007), 51-59, describes the formation of a chitosan gutter layer.

The gas permeable layer can be introduced onto the support by castingthe polymer in question (e.g. PDMS) in liquid form onto the support. Thethickness of the layer can be 100 nm to 100 μm, preferably 250 nm to 10μm, especially 300 nm to 5 μm. Ideally the layer should be as thin aspossible whilst still providing the necessary mechanical strength.Thereafter the membrane can be formed on the permeable layer asdescribed above.

Tests have shown that membranes of the invention can be used for atleast 800 hours without any significant loss of activity and this formsa further aspect of the invention.

The membranes of the invention operate most effectively when they arehumid. Before use of the membranes therefore, they may be swelled in thepresence of water, e.g. in the form of vapour. Ideally, the membranes ofthe invention should operate in a humid environment, e.g. at least 75%relative humidity.

The process for the preparation of the membranes of the inventiontherefore preferably further comprises a step of contacting the membranewith water, e.g. with water vapour and/or operating the membrane in ahumid environment.

It is envisaged that the presence of water vapour in the membranefacilitates carbon dioxide transport across the membrane.

Gases which can be separated from gas mixtures using the membranes ofthe invention include carbon dioxide with various components such asnitrogen, methane, carbon monoxide, oxygen, volatile organic compoundsor hydrogen. Separation of mixtures involving hydrogen is alsoenvisaged. These gases can occur in any circumstance such as inindustrial and domestic gas streams. In use, the gas mixture to beseparated will typically flow across the membrane under pressure. Thetemperatures employed can vary but typically at temperatures are in therange of 10 to 90° C., preferably at 20 to 65° C. It is possible to workat even higher temperatures however and separation at temperatures ofgreater than 100° C. may offer improved results.

Preferably, the membrane is used to separate carbon dioxide fromnitrogen or methane. In this latter regard, the membranes of theinvention may therefore have applications in the field where these gasesare present in mixtures such as flue gas, biogas upgrading or possiblysweetening of natural gas.

The pressure at which the gas mixture is applied to the membrane isimportant as it affects the flow across the membrane and potentially theselectivity thereof. Feed pressures may therefore be in the range of 1.0to 100 bars, e.g. 1.0 to 20 bars, especially 2 to 15 bars. The membranesof the invention are especially useful for feed pressures of at least 3bars, preferably at least 4 bars, especially at least 5 bars, moreespecially at least 10 bars. Feed pressure can be in the range of 1 bar(typical flue gas)—80 bar (typical natural gas). The membrane of theinvention is most useful for application at pressures below 10 bar.

The membranes of the invention preferably exhibit selectivities of atleast 20, more preferably at least 50, especially at least 100, mostespecially at least 150. Selectivity is measured as described in theexamples.

Permeance values in m³(STP)/m²·h·bar are preferably at least 0.1,preferably at least 0.2, especially at least 0.3, most especially atleast 0.4, e.g. 0.4 to 1.0

The invention will now be further described with reference to thefollowing non-limiting examples and figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a diagram of the experimental set up used to measurepermeance.

FIG. 2 shows permeance variation with feed pressure for cross-linked vsthermally treated membranes.

FIG. 3 shows selectivity variation with feed pressure for cross-linkedvs thermally treated membranes.

FIG. 4 shows permeance variation of the composite membranes of theinvention with pressure.

FIG. 5 shows selectivity variation of composite membranes of theinvention with pressure.

FIG. 6 shows permeance variation of the support.

FIG. 7 shows permeance variation of the composite membranes of theinvention using thermally treated membranes vs non strengthenedmembranes.

FIG. 8 shows selectivity variation of the composite membranes of theinvention using thermally treated membranes vs non strengthenedmembranes.

FIG. 9 shows the water uptake contrast between high and low Mw PVAmmembranes.

PERMEATION TESTING

Permeance of the membranes was measured with an apparatus equipped witha humidifier, see FIG. 1. FIG. 1 shows an experimental setup for gaspermeation measurements. The chosen gases may be mixed in any ratios ina gas flow line A, in which flow, pressures and temperature arecontrolled. The gas mixture is lead to humidifiers in tanks 1 where itbubbles through water, and then to a membrane separation cell 2. Eitherthe retentate stream, or the permeate stream may be lead to a gaschromatograph (GC) 4 for analysis of the composition. The water excessis removed by a liquid separator 6 before going to the GC.

The various gas flows are controlled by valves and flow controllers. Theabbreviations FI, FC, PI and PC in circles are flow indicator (FI), flowcontroller (FC), pressure indicator (PI) and pressure controller (PC),respectively. The use of this equipment will be familiar to the skilledperson.

Premixed gas with a molar composition of 10% CO₂-90% N₂ was used as feedand methane was used as a sweep gas, both feed and sweep gases beinghumidified by passage through two water bubblers. The permeate side wasmaintained at atmospheric pressure and the total flow (permeate plussweep) was measured with a soap bubble meter. The RH % was controlled bytwo bypass valves controlling the ratio of dry/wet gas (V₄ and V₁₄). Thecompositions of the permeate and feed were analysed continuously by amicro GC (gas chromatograph) Agilent 3000 equipped with two thermalconductivity detectors (TCDs) and two columns, Molecularsieve and PlotQ. A liquid separator was installed before GC sampling valve in order toprevent the moisture penetration inside GC. The permeance of CO₂ and N₂was calculated using complete mixing model, from the total permeate flowrate measured with a soap bubble meter, feed and permeate pressure andthe gas compositions of feed and permeate gas measured by GC.

$Q_{i} = \frac{J_{i}}{A( {{x_{r,i}p_{h}} - {x_{p,i}p_{1}}} )}$where Q_(i) represents the permeance (m³(STP)/(m² bar h) of component i(CO₂ or N₂), Ji represent the flux m³(STP)/h, A the membrane area (m²),x_(r,i) and x_(p,i) molar concentration on feed and permeate siderespectively (mol %) and p_(h) and p_(i) absolute pressure on feed andpermeate side (bar).

The selectivity α of CO₂ over N₂ was calculated using the permeanceratio of the two gases when permeating together in a mixture (10%CO₂-90% N₂) without excluding the reciprocal coupling effect betweengases.

All experiments were conducted at a constant temperature of 25° C. andthe pressure difference between the feed and the permeate sides was 1-15bar.

Results for the membranes of the invention with a 10% CO₂/N₂ mixture arepresented in the figures.

Example 1 PVAm Purification (Pre-Treatment)

PVAm with a molecular weight of 340 000 was purchased from BASF and is alinear polymer (Scheme 1) having more than 90% of the amide groups ofpolyvinylformamide (PVAF) hydrolyzed to amino groups—more than 90% is inPVAm and 10% is in PVAF form. The polymer is obtained by polymerizationof the vinylformamide (VFA) monomer to polyvinylformamide andconsequently hydrolyzed under acid or basic conditions to formpolyvinylamine.

The PVAm polymer was provided in the form of aqueous solution with pH=8.Further purification of the polymer was carried out in order to removepossible traces of vinylformamide monomer or other impurities and tomaximize the hydrolysis degree.

The solution was purified in successive steps:

(I) re-precipitation in acetone and/or acetone and ethanol blend:

(II) washing with acetone

(III) filtration

(IV) drying until constant weight and

(V) re-dissolution in distilled water.

This procedure was repeated several times and the resulting polymersolution was completely hydrolysed by acidic hydrolyse in presence ofHCl 5M solution and was re-precipitated in the form of PVAm.HCl(protonic form).

Example 2

Comparative PVAm (Mw 80,000) was prepared by the Hofmann reaction ofpolyacrylamide based on Hiroo Tanaka and Ryoichi Senju, Preparation ofpolyvinylamine by the Hofmann degradation of polyacrylamide, Bulletin ofthe chemical society of Japan, 49, 10 (1976) 2821.

Example 3 Flat Sheet Membrane Preparation

Composite asymmetric membranes were prepared by casting a solution ofPVAm onto asymmetric porous polysulfone supports. The polysulfone poroussupport was either MWCO 20000 or 50000 respectively. The supports werewashed in advance with large amounts of distillate water. The desiredthickness of the PVAm layer was controlled by pouring a known volume ofsolution into a confined circular surface with known area on thesupport. The calculated and resulted PVAm membrane thickness was 1.2 μmon dry basis for all membranes (unless otherwise mentioned). Thethickness of the dry membranes was confirmed from Scanning ElectronMicroscopy (SEM) cross section pictures.

The membranes were dried at 45° C. for 90 minutes and then kept at roomtemperature for 24 hours. Membranes were used as such or treated asdescribed in Example 4.

Example 4 Cross-Linking/Thermal Treatment

For the cross linking experiments, ammonium fluoride solution was used.The membranes were cross linked with a known volume of aqueous solutionsof NH₄F having concentrations of 1M to 3M. Subsequently the membraneswith NH₄F were heat treated at 90° C. for one hour and kept for 24 hoursat room temperature before permeation tests.

Thermal treatment was carried out by annealing the obtained membranes at90° C.-125° C. for one hour in absence of ammonium fluoride.

Example 5 Hollow Fibre Type Membrane Preparation

The membranes were prepared using hollow fibre membrane porous supportsformed from polysulfone (PSf) or polyphenylene oxide (PPO). Twoprocedures for coating outside and respective inside hollow fibre weredeveloped. For outside coating, the membranes were prepared by dipcoating technique: the porous support was immersed in PVAm aqueoussolution followed by drying at 45° C. The procedure was repeated until adefect free film of PVAm was formed on the surface of the poroussupport. The resulted hollow fibres were thermally treated (annealed) at90-125° C.

For inside coating, aqueous PVAm was circulated for 30 minutes insidethe hollow fibre lumen, followed by drying as in the case of outside dipcoating.

In more detail, supports were mounted in a module and coating inside wasperformed in steps:

A vacuum pump was connected to the module, evacuating the outside of thehollow fibre from atmospheric pressure to 30 mbar (see T. Kouketsu etal, supra).

The coating solution was circulated for 30 minutes into the bore side ofhollow fibres

The excess solution was removed by nitrogen purge

The coated hollow fibres were dried and the temperature and the dryingtime were dependent on the boiling point of the solvents used

More than one layer of PVAm can be applied

The following day, heat treatment of the membrane was performed as peroutside coating.

Example 6 Membrane Testing: Water Uptake (Swelling Degree) Experimentswere Measured on the Non Strengthened Flat Membranes

Maximum water uptake capacity of PVAm by weight was investigated usingfully humidified nitrogen. Water uptake by membranes represents a keyparameter for facilitated transport where the reaction between CO₂ andamino groups takes place in presence of water. High water uptakecapacity is directly related to density of polar groups (amino groups)per volume. The degree of polymeric chains entanglement will dictate thewater “holding” capacity of polymer. Water has a positive effect bycatalysing the reaction between amino groups and the CO₂, provides atransport medium for the CO₂ to the amino group reaction sites andincreases polymeric chains mobility. Water uptake could have had thenegative effect of loosening the PVAm structure to the point ofdisrupting completely the chain packing but that is surprisingly notobserved. The effect of swelling on the mechanical stability of thePVAmHM membrane is counter-balanced by the dense chain packing.

The Mw influence on water uptake was investigated by comparing the wateruptake of PVAmHM (340000 Mw) to that of PVAmLM (80000 Mw). As can beseen from FIG. 9 the water uptake (SD %) increases fast with time,approaching a plateau after 6 hours. The experiments were carried outuntil no change in the mass of swollen polymer was observed (22-24 h).PVAmHM presented a maximum water uptake at equilibrium of 31.4% andPVAmLM 24.7% by weight. From repeated measurements PVAmHM presents inaverage 23% higher water uptake than PVAmLM. This fact can be explainedby the PVAmHM's higher Mw (longer polymer chains), providing a moreentangled structure and higher density of polar sites per polymervolume. The higher water uptake of PVAmHM in comparison with PVAmLM canbe interpreted in terms of better structural film integrity in absenceof crosslinker under humid conditions.

Example 7 Effect of Molecular Weight on NH₄F Cross-Linked Flat Membranes

The higher molecular weight PVAm ensures a denser packing of the aminogroups and higher water uptake leading to higher selectivity andpermeance compared to low molecular weight PVAm.

Four times higher Mw of PVAm (340 000) allows the use of a poroussupport with higher MWCO that increases permeance without affecting themechanical strength of the composite membranes.

Using larger pore size not only decreases the mass transfer resistancetowards CO₂ but it changes the support separation mechanism itself. Anultrafiltration membrane with low pore size (MWCO) may presentselectivity towards the N₂ via Knudsen diffusion and not towards CO₂.

Example 8 Effect of Support

We investigated the influence of the support as a separate membraneunder normal operating conditions in order to clearly accesses itscontribution to the overall membrane performance especially in the humidconditions. FIG. 6 show the separation performance of the poroussupports tested with mix gas feed. As can be seen the dry PSf has a verylow CO₂ permeance and selectivity. The porous supports act as membranespresenting selectivity when using humidified gases. It is clear fromFIG. 6 that high MWCO support presents higher CO₂ permeance especiallyat lower pressure ranges.

Example 9 Crosslinking

FIGS. 2 and 3 show the effect of the NH₄F solution concentration usedfor crosslinking at 90° C. High Mw provides a more entangled structureof the polymeric chains (equivalent of a crosslinking) providing betterfilm mechanical properties in absence of a crosslinker.

Example 10 Effect of Thermal Treatment

FIGS. 7 and 8 show the effect of thermal treatment on one of the flatmembranes of the invention. The permeance and selectivity are onlyslightly better than for the non strengthened analogues suggesting thisstep is barely necessary but for pressures above 10 bars, the nonstrengthened membranes lack sufficient mechanical strength,

Example 11 Effect of Feed Pressure

FIGS. 4 and 5 shows the effect of the applied feed pressure on PVAmHM(340000 Mw) composite membrane CO₂ permeance and selectivity. Thedecrease in CO₂ permeance and CO₂/N₂ selectivity with the increase infeed pressure (CO₂ partial pressure) correlates with the CO₂ facilitatedtransport mechanism and it is explained by the saturation of fixed sitecarrier amino groups. It can be observed that the PVAmHM cast on moreporous PSf support presents higher permeance up until 10 bars due to thelarger pore size of the support. The difference in permeance isattributed to the effect of the porous support acting as a resistance inseries with the selective PVAm layer. The CO₂ permeance of bothmembranes becomes similar at pressure higher than 10 bar due to apossible compression of the more porous support. The higher CO₂/N₂selectivity up until 3 bar is explained by the higher CO₂ permeance ofthe 50000 MWCO support.

Example 12

Each flat membrane was continuously operated between 30 and 60 daysshowing stable performances by swinging feed relative humidity (RH %)between 0% to 100%, the pressure between 1 bar and 15 bar. A maximumpermeance of 0.6 m³ (STP)/(m² bar h) and selectivity of 200 at 1.1 barfeed pressure were obtained for PVAmHM 1.2 μm thickness membranes castedon 50 000 MWCO PSf without crosslinking.

Example 13 Effect of Solvent

Different solvents provide different volatilities and the quality ofvery thin films can be affected by drying conditions, especially forhollow fiber coating or large flat membranes. Maximum permeance andselectivity using different solvents, for a flat composite membrane of1.2 μm thickness without crosslinking cast on PSf 50000 MWCO as supportis presented in Table 1. The operating conditions were 10% CO₂-90% N₂gas mixture at 2 bar as a feed. Using different solvents not only as amean of casting a polymer in a shape of a film but mainly as an agentmodifying the structure of the PVAm film provides different separationproperties. The PVAm film structure and separation properties changesfunction of the PVAm solubility in different solvents, evaporating rateof the solvents, pH of the solvent (changing the character of PVAm fromacid to base and implying the reactivity change of —NH2 groups) andinteraction with the porous support (hydrophobic-hydrophilic character).

TABLE 1 Effect of casting solvent 10% water- Type of solvent Water 90%formamide Permeance 0.6 2.1 m³(STP)/m² bar hr CO₂/N₂ selectivity 204 388

Example 14 Gas Permeation Results for Hollow Fibre Type of Membrane

Table 2 presents the results of gas permeations using hollow fibre (HF)type membrane with a selective separating layer of PVAm of approx. 1 μm.If CO₂/N₂ selectivity does not differ to a great extent it may beattributed to the PVAm selective layer, while CO₂ permeance is affectedto a great extent by the porous support. A porous support with ananometer dense top layer such as polyphenylene oxide represents a moreconvenient choice in terms of gas separation and mechanical strength dueto the intrinsic material properties.

TABLE 2 Effect of different HF support Type of HF PSf outside PPOmembrane coated PSf inside coated outside coated OD/ID mm 0.6/0.3 1/0.60.5/0.3 Permeance 0.1 0.016 1.46 m³ (STP)/m² bar hr CO₂/N₂ selectivity140 157 191 OD outer diameter, ID inner diameter

What is claimed is:
 1. A process for the formation of a compositemembrane suitable for separating a gas from a gas mixture comprising:(I) obtaining a polyvinylamine (PVAm); (II) hydrolyzing said PVAm underconditions of acid to form a pre-treated PVAm; (III) forming a solutionof said pre-treated PVAm in a solvent; (IV) casting said solution onto asupport to form the composite membrane comprising the support and thesolution of pre-treated PVAm in solvent; and optionally (V) thermaltreating said composite membrane, wherein the PVAm in the compositemembrane is not crosslinked.
 2. A process as claimed in claim 1 whereinsaid support is porous.
 3. A process as claimed in claim 1 wherein saidsolvent comprises methanol, ethylene glycol, formamide or mixtures ofone or more of said solvents with water.
 4. A process as claimed inclaim 1 wherein the pH of the solution is at least
 6. 5. A process asclaimed in claim 1 wherein said PVAm in the composite membrane has amolecular weight of at least Mw 50,000 and wherein the compositemembrane is heated to a temperature in the range 50 to 150° C. in step(V).
 6. A process as claimed in claim 1 wherein said PVAm in thecomposite membrane has a molecular weight of at least Mw 50,000 andwherein the membrane is heated to a temperature in the range 80 to 120°C. in step (V).
 7. A process as claimed in claim 1 wherein said PVAm inthe composite membrane has a molecular weight of at least Mw 50,000 andis non-crosslinked.
 8. A process as claimed in claim 1 wherein said PVAmin the composite membrane has a molecular weight of at least Mw 50,000and wherein the support has a molecular weight of at least 20,000.
 9. Aprocess as claimed in claim 1 wherein said PVAm in the compositemembrane has a molecular weight of at least Mw 50,000 and said supportis microporous.
 10. A process as claimed in claim 1 wherein the PVAm inthe composite membrane has a Mw of at least 100,000.
 11. A process asclaimed in claim 1, wherein said support is polytetrafluoroethylene(PTFE), polypropylene (PP), sulphonated polysulfone (PSf),polyvinylidene fluoride (PVDF), polyimide (PI), polyether imide (PEI),aliphatic polyamide, polyetheretherketone (PEEK), or polyphenylene oxide(PPO).
 12. A process as claimed in claim 1, wherein a carbon dioxidepermeable layer separates the PVAm in the composite membrane from thesupport.
 13. A process as claimed in claim 12, wherein said carbondioxide permeable layer is polydimethylsiloxane (PDMS), PVA(polyvinylalcohol) or chitosan.
 14. A process as claimed in claim 1wherein the pretreated PVAm comprises less than 5 wt % polyvinylamide.